This invention relates to a falling film evaporator. In more detail, the invention relates to a falling film evaporator of simple construction incorporating novel means for insuring that all heating surfaces are wetted as efficiently as possible. In still more detail, the invention relates to a falling film evaporator suitable for use with salting and severely scaling liquids. The invention more particularly relates to a falling film evaporator useful for the evaporation of solutions containing plutonium and minor amounts of organic solvents with vapor pressures that allow removal by steam stripping.
While presently available evaporators can be used for the evaporation of solutions containing plutonium, none are known which combine the virtues of simplicity and low cost with the attributes desired of an evaporator to be used for the purpose. For example, since the solutions are heat sensitive and prone to contamination from materials of construction, the evaporator should provide a short residence time for the solution to be concentrated. In addition, it should be possible to evaporate a solution containing a high percentage of salts therein, in some cases to the stage of salt deposition, with no clogging of the evaporator. In addition, since such solutions contain a minor amount of organic solvent, it would be desirable to remove the organic solvent at the same time as the water is removed. It is to be noted that many presently available evaporators can remove the water but reflux the organic phase. Such evaporators will not efficiently strip organic solvent from the feed and a separate steam stripping operation must be employed.
Falling film evaporators are known to be very useful for evaporating heat-sensitive materials particularly under a vacuum. The principal problems with falling film evaporators are feed distribution and complexity of equipment (rotary wipers, etc.). To optimize operation of falling film evaporators, it is essential that all heated surfaces be wetted as completely as possible. Recirculation of the liquid and/or various proprietary distributors may be used to help ensure this, but other problems such as complexity of equipment (e.g. failure of working parts) and inefficient organic stripping may arise. In addition, evaporators of this type are generally unsuited to salting or severely scaling liquid due to the possibility that tubes will clog, bearings will freeze, plates will plug, etc.
In the processing of nuclear fuel employing TBP (tributylphosphate) as extractant, solutions are obtained containing plutonium nitrate and small quantities of TBP. Product solutions are obtained containing a relatively high concentration of plutonium and waste solutions are obtained containing a low concentration of plutonium. The evaporator of the present invention can be used to concentrate either of these solutions. The evaporator of the present invention was designed with these solutions in mind but would, of course, be useful with other feed materials.
Another important feature of this tube type evaporator is that the maximum diameter can be limited to 6 inches I.D. (inside diameter). Hence fissile materials like plutonium can be concentrated while assuring critical mass safety, i.e., the geometry is such that the leakage of neutrons is always greater than the buildup of neutrons and the system cannot go critical. By lengthening the tubes or having a series of tubed sections, the desired capacity can be attained, equipment is simple (no moving parts to worry about) and critical mass safety is assured.